RCD testing problems on a TT system

[pencilpusher]

I am testing my garage which has a TT system. I have a 30mA RCD as a main switch but am having problems with RCD testing using my Megger 1720. I have a Ze of 196 ohms which I know is OK so began testing the RCD. When I come to a x5 test it will not test because it tells me I have a touch voltage of 51V or so. I believe the limit is 50V. Then the next minute it did a full set of tests on an auto setting. The next minute it would not test again.

Help!

Thanks

PP

EDIT: The x1 test came out at 29ms so should be fine on a x5 anyway. I just dont understand why sometimes it will test at x5 and the next minute it won't.

 

[malcolmsanford]

Re: RCS testing problems on a TT system

Well you really answered it yourself a meter will not test with a touch voltage of over 50v,

As the voltage supply fluctuates on it's cycle you will find that it dipped below the threshold and tested.

Now though 198 ohm is technically acceptable, in reality it is quite poor especially if it wasn't stable and that would be my first job, to perhaps get a lower and more stable reading.

Also did you have all loads isolated when you did the tests, if you tried to do it while loads were connected this to will add to your problems

 

[pencilpusher]

Re: RCS testing problems on a TT system

Thanks Malcolmsanford. The loads were isolated yes. The 196 is stable but I take your point. I have a simple earthing rod next to the garage.

 

[IQ Electrical]

As Malcolm said, it sounds like your Ra value (not Ze) is not stable, thereby allowing the voltage to rise to a value exceeding 50V and causing the meter to abort the test.

 

[Guest123]

I'd agree with Malc & IQ also.

At 5 x IDN, the test current is 150mA. Divide 50V into this and you'll get around 330ohms. An unstable reading may rise enough to cause the meter to abort the test, plus the meter tolerances may be slightly out.

 

[pencilpusher]

Thanks to all. I was going to add that I have 196 ohms and that it was under 333 ohms which would equate to 50V. I cannot understand why I am getting 50V plus. Surely I would expect 0.150 amps (5x) x 196 ohms = 29 V?

 

[IQ Electrical]

Thanks to all. I was going to add that I have 196 ohms and that it was under 333 ohms which would equate to 50V. I cannot understand why I am getting 50V plus. Surely I would expect 0.150 amps (5x) x 196 ohms = 29 V?

Exactly, if the 196 Ohm Ra value was accurate and constant, that's what you would see but in practice, there are many variables, including as Lenny mentioned, meter accuracy.

The only accurate way of measuring Ra is by using an earth electrode resistance tester which has a vastly superior accuracy compared to the EFLI method that we mostly use.

The EFLI method is accepted because accuracy on RCD protected TT systems isn't such an issue because of the higher values normally encountered.

 

[Graeme Harrold]

Ive got a fluke and usually have problems testing x5 when Ze is above 120-150ohms, it usually fails on the voltage rising above 50V within the max 40ms required. Try sticking in another rod to drop the Ze.

 

[Deleted member 9648]

Ive got a fluke and usually have problems testing x5 when Ze is above 120-150ohms, it usually fails on the voltage rising above 50V within the max 40ms required. Try sticking in another rod to drop the Ze.

Or just chuck a bucket of water or three on it just before you test.....a lot simpler.

 

[pencilpusher]

Or just chuck a bucket of water or three on it just before you test.....a lot simpler.

Of course! :smilielol5:

 

[Graeme Harrold]

Not the way I used to use to keep a genny rod wet.............:stuart:

 

[topquark]

Not the way I used to use to keep a genny rod wet.............:stuart:

I take it you used to drink the water _

 

[Guest123]

I would look at the MFT as megger seem to having quite a number of issues with their new model.

 

[pencilpusher]

I would look at the MFT as megger seem to having quite a number of issues with their new model.

Lenny - What are my options in relation to this? It was new in June 2010.

 

[jaresquire]

Not the way I used to use to keep a genny rod wet.............:stuart:

Having seen (on t'internet) various dead vagrants beside a live rail I would be a bit nervous doing it the way you imply!